Trace gas analyzers can require periodic validation of long term fidelity of the concentration measurements they generate, for example with respect to the performance of an analyzer relative to factory calibration or relative to a standard which is traceable to a national or international bureau of standards (e.g. including but not limited to the National Institute of Standards and Technology). Currently available solutions for in-field measurement validation typically include the use of permeation tube devices or calibrations performed using reference samples provided from a compressed gas cylinder.
Frequency stabilization of a tunable laser light source can be critical for quantitative trace gas absorption spectroscopy. Depending on the operational wavelength, a tunable laser source such as a diode laser can typically exhibit a wavelength drift on the order of a few picometers (on the order of gigahertz) per day to fractions of picometers per day. A typical trace gas absorption band linewidth can in some instances be on the order of a fraction of a nanometer to microns. Thus, drift of the laser light source can, over time, introduce critical errors in identification and quantification of trace gas analytes, particularly in gas having one or more background compounds whose absorption spectra might interfere with absorption features of a target analyte.
Permeation tube based validation systems are generally costly and complex and typically require very precise control of temperature and gas flow rates and elimination of temperature gradients across the permeation tube to provide an accurate result. Aging and contamination of permeation tube devices can alter the permeation rate over time, thereby causing a change in the validation measurement reading and potentially rendering the validation inaccurate over time. This problem can be addressed, albeit at potentially substantial expense, by frequent replacement of the permeation tube device. Further challenges can arise in the replacement of permeation tube devices in the field, as it can be difficult to correlate the trace gas concentration generated by a replacement permeation device to a bureau of standards traceable analyzer calibration. Permeation-based validation systems can also require a significant amount of carrier gas and analyte gas to prepare the validation gas stream. Permeation-based devices generally are not feasible when very reactive or corrosive gases are involved. Furthermore, permeation-based devices generally cannot accurately prepare low concentration (e.g., less than 10 parts per million and particularly on the order of parts per billion or smaller) validation streams for trace analyte measurements. A validation stream should advantageously remain accurate over a practical operating temperature range. Extreme temperature sensitivity of permeation devices can be a key challenge. For example, temperature changes of as little as 0.1° C. can cause moisture concentration changes of magnitude greater than ±10% of the nominal validation concentration, which is generally not acceptable for in field analyzer validation.
Validation using a reference gas of known concentration provided from a compressed gas cylinder can be used for gas chromatograph validation applications. Such an approach can be substantially more costly with a spectroscopic measurement. A reference gas measurement can involve gas flow rates through a sample measurement cell at rates of, for example, approximately 0.1 to 3 liters per minute, which is multiple orders of magnitude greater than the typical flow rates of micro-liters per minute used in gas chromatographs. Reference gas blends provided in compressed gas cylinders can be difficult or impossible to obtain, especially in remote areas of the world where many natural gas processing, petrochemical, chemical, and refining plants are located. Shipping of pressurized gas cylinders can be costly and can take a very long time because pressurized gas cylinders generally cannot be shipped on airplanes. Additionally, reference gas cylinders can require heating blankets or placement inside temperature controlled cabinets, housings, etc. to prevent diurnal temperature fluctuations from rapidly degrading the certified reference composition in the cylinder. In addition carrier gas and trace analytes have been found to not mix uniformly, e.g. at typical gas cylinder pressures of 50 psi (pounds per square inch) to 3000 psi, without mechanical agitation or heating. As a result, even a reference gas mixture that is gravimetrically certified upon original preparation (e.g. by use of suitable bureau of standards traceable weights and scales) can produce varying trace gas concentrations in the gas withdrawn from the cylinder over time, thereby creating erroneous, changing concentration readings of the analyzer during successive validation attempts.
Even with such precautions, however, a pressurized cylinder containing a reactive trace gas will typically maintain a stable, reproducible reference gas concentration for only a few months at most, due to reactions of the trace gas with the cylinder. Reactions with cylinder walls can be a significant issue for many reactive trace gases, including but not necessarily limited to H2S, HCl, NH3, H2O, and the like. It can be especially difficult to prepare accurate moisture blends that remain stable for a period longer than 6 months. At present, certified and traceable reference gas blend in pressurized cylinders that reliably provides a moisture content of less than about 10 ppm with an accuracy better than approximately ±10% are not available. Thus, instrument validations for analyzers capable of measuring moisture levels of less than about 1 ppm, for example in liquefied natural gas, dry cracked gas, hydrogen, nitrogen, oxygen, air, ethylene, propylene, olefins propane and butanes, can be extremely difficult. As an example, this lack of a suitable moisture reference gas mix for concentrations <10 ppm currently presents a very significant operational challenge for production of liquid gas. Typically, natural gas liquefication trains need to reliably maintain moisture levels well below 1.5 ppm to mitigate icing of the liquefication equipment. Undetected moisture excursions to levels above about 1 ppm generally lead to icing of the equipment. A single instance of needing to thaw the gas liquefication equipment to restore productive operations can readily result in an operating loss in excess of $5,000,000.
Production of ethylene and propylene, the basic building blocks for the vast majority of plastics used in daily life, carries requirements to maintain trace impurity levels well below 50 ppb to prevent production of inferior quality poly-ethylene and poly-propylene. These impurities can include, but are not limited to, NH3, H2O, C2H2, CO2 and CO. In general, bottled gas mixtures cannot provide accurate, bureau of standards traceable validation for such low concentration measurements. Permeation tube validation technology is not well suited to providing trustworthy validation results for ethylene and propylene contamination measurements either. In addition to the extreme requirements for temperature stability and flow control, permeation tube devices are generally incapable of reliably providing trace gas concentrations below 10 ppm. Typical optical and TDL trace gas analyzers that measure below 50 ppb cannot support accurate measurement of trace gas levels greater than approximately 10 ppm at the same time.